1. Field of the Invention
The invention relates to thin film gas sensors and somewhat more particularly to thin film semiconductor gas sensors having a metal oxide semiconductor sensor layer whose electrical resistance changes in dependence upon the nature and concentration of a gas being detected and having a heating element required for its function integrated into such sensor.
2. Prior Art
Selective gas sensors based on a metal oxide semiconductor are known, for example, from German Offenlegungsschrift No. 27 35 222. In this gas sensor, the metal oxide semiconductor comprises a layer of tin oxide (SnO.sub.2) and detects the presence and concentration of ethyl alcohol in air. The operative principle of such gas sensor comprises the phenomena that with the absorption and reaction of a specific reactive gas from air, a change of electrical conductivity in the sensor layer occurs and is monitorable. The sensitivity of such a gas sensor is measured by the relative change of electrical conductivity per unit of gas concentration in air. This type of measuring arrangement is known from the above-referenced German Offenlegungsschrift No. 27 35 222.
Semiconductor gas sensors in the form of relatively thin films of metal oxide semiconductors, require, for their function, an elevated operating temperature, which, depending upon the construction of the sensor, its chemical composition and the medium to be detected, is at least 150.degree. C. and preferably in the range of about 300.degree. to 400.degree. C.
U.S. Pat. No. 3,865,550 discloses a semiconductor gas sensor having a heating element in the form of a coiled filament fused into a glass bead. The metal oxide semiconductor sensor film is positioned on such glass bead.
The gas sensor disclosed in the earlier-referenced German Offenlegungsschrift No. 27 35 222, on the other hand, is provided with a coiled filament, for example, composed of a chromium-nickel wire, which is passed through a ceramic tube. The metal oxide semiconductor sensor film is applied on the exterior of this ceramic tube.
Other than the advantages that such coiled filaments are simply producable manually and are exchangeable as desired, these structural shapes have a series of disadvantages. For one thing, these types of structures are produceable only to a small extent automatically and thus require a large amount of manual labor, which is uneconomical. For another thing, such coil filaments, for the most part, are composed of platinum (because other metal heating wires are not as resistant to corrosion) and thus require a considerable outlay of noble metal.
German Offenlegungsschrift No. 29 33 971 (assigned to the instant assignee) discloses a gas sensor element which includes an insulating substrate of oxidized silicon, saphire or spinel having a heating layer composed of a vapor-deposited layer of nickel-chromium alloy or platinum on the substrate and a contact layer composed of platinum or gold/palladium vapor deposited on the nickel-chromium layer.